1. Field of Invention
This invention relates to a flaw detecting method and an apparatus therefor suitable for detecting flaws in a strip while being carried.
Further, this invention relates to the manufacture of a hot-rolled sheet or a cold-rolled sheet, and, in particular, an inspecting method and a manufacturing method of a sheet. This invention also relates to a manufacturing equipment for a sheet.
2. Description of Related Art
In the area of manufacture and working of metallic and nonmetallic strips such as steel strips, there is a demand for a technique of online detection of flaws contained in strips to achieve quality control and quality assurance. The need is particularly for an apparatus capable of detection of internal fine flaws, and such an apparatus is generally known as a testing apparatus.
There is an another technique comprising, analyzing a specimen sampled from rolled steel strip, estimating the quality level of whole steel strip on the basis of the results of analysis, as disclosed in Japanese Unexamined Patent Publication NO. 08-184537 and NO. 10-185902. However, these techniques have no reliability in their application to the evaluation of steel strip with few internal flaws such as steel strip for cans, because the probability of sampling of internal flaws is extremely low.
Testing apparatus capable of continuously testing over the entire volume of a strip (such as a steel strip) that is carried continuously mainly include testing apparatus based on the magnetic leakage flux testing technique and testing apparatus utilizing ultrasounds.
The magnetic leakage flux testing technique comprises magnetizing a strip (typically a ferromagnetic metallic strip) by a magnetizing device and detecting leakage of the magnetic flux caused by flaws using a magneto-sensitive element such as a Hall-effect element, coil, or magnetic diode.
It is, however, impossible to test a strip having a thickness of over approximately 0.5 mm by the magnetic leakage flux testing technique. For a strip having a thickness as large as that of a hot-rolled steel sheet, the ratio (flaw cross-sectional area/steel sheet cross-sectional area) becomes smaller, and this makes it difficult for the magnetic flux to leak to the surface.
The magnetic leakage flux is rapidly attenuated in inverse proportion to the distance from the strip surface. It is therefore necessary to control upward and downward fluctuations of the strip pass-line within .+-.0.1 mm, and limit the gap between the detecting head and the strip surface within 0.5 mm. Because of the necessity of such a strict gap control, it is difficult to continuously and stably test the strip in transfer. Particularly, at a high carrying speed of the strip, the gap control is more difficult.
Another problem of the magnetic leakage flux testing technique is that a false detection can easily occur because of many noise factors. The magnetic leakage flux testing technique has a further limitation that is impossible to obtain accurate information of the shape of a detected flaw.
The ultrasonic testing technique comprises applying ultrasounds into a strip, thereby detecting reflection or shadow caused by internal flaws. Because it is possible to provide a large gap between the strip surface and the detecting head as compared to the magnetic leakage flux testing method, and detect flaws even in a thick strip, the ultrasonic testing technique is considered more suitable for the continuous testing of general strips.
There is also known a contacting ultrasonic testing technique known as the lamb wave testing technique. This technique is based on the detection propagating the lamb wave in the width direction of the strip through rolling contact of a wheel search unit (detecting head) with the strip surface. A disadvantage of this technique is its low detectability and the presence of a dead zone in the strip thickness direction. Further, it is practically impossible to test a wide range of the strip continuously in high speed transfer.
Because the lamb wave testing technique is a contact type technique, the probe may sometimes become bound and further, the medium between wheel search unit and the strip cannot be stably supplied at high transfer speeds of the strip. Thus, the carrying speed of the strip is limited within a low range. The lamb wave testing technique has another risk of bursting of the wheel.
An immersion (or soak or dip) testing technique, such as water immersion testing, is a non-contact testing technique and is free from the problems as described above. That is, there is available an advantage of a slight effect of fluctuations of the pass-line upon transfer of the strip.
For immersion testing using ultrasounds for the propose of detecting flaws such as inclusions for the entire volume of a strip such as a rolled metallic sheet, the following two techniques are proposed, having different arrangements of the ultrasonic probe (detecting head):
(1) A technique comprising testing a rolled metallic sheet while carrying the same, by arranging a plurality of ultrasonic probes in the width direction of the rolled metallic sheet to be tested, as disclosed in Japanese Unexamined Patent Publication No. 60-78345; and PA1 (2) A technique of testing a rolled metallic sheet while carrying the sheet by scanning the rolled metallic sheet in a direction substantially at right angles to the carrying direction of the sheet with ultrasonic probes, as described above, arranged in the width direction of the rolled metallic sheet. PA1 T1: a wave that is transmitted from the transmitting probe array, and reaches the receiving probe array. PA1 T2: a wave that is transmitted from the transmitting probe array, reflected at the back surface of the strip to be tested, reflected at the upper surface of the strip to be tested, and reaches the receiving probe array. PA1 F1: a flaw echo that is part of ultrasound transmitted from the transmitting probe array, reflected at the upper surface of the flaw, reflected at the upper surface of the strip to be tested, and reaches the receiving probe array. PA1 F2: a flaw echo that is part of ultrasound transmitted from the transmitting probe array, reflected at the back surface of the strip to be tested, reflected at the back surface of the flaw, and reaches the receiving probe array. PA1 (1) Because the flow of manufacturing processes branches off after cold rolling in accordance with the plating method or the like, it is necessary to install a testing apparatus for each of the lines, leading to a higher cost. PA1 (2) Even if a flaw is detected, possible uses of the cold-rolled sheet are limited after detection. When a flaw is discovered after finishing into a product size, for example, the destination of the sheet after testing cannot be changed. The defective sheet thus is rejected as a scrap, resulting in a large decrease in yield, leading to the economic disadvantage. PA1 (3) Feeding back a factor causing the flaw requires much labor and time. That is, in order to feed back the factor, it is necessary to discover a process in which the flaw has occurred and the source of occurrence, and for this purpose, it is necessary to investigate shapes of flaws and relate the result with the operating conditions of each process. It is, however, difficult to clarify sources of flaws because many processes are present between the process in which the flaw has occurred and the process in which the flaw has been detected, and the sequence of works-in-process depends upon circumstances of treating timing on each line. Further, the investigation takes much time, so that a response (action) to avoid the occurrence of the flaw cannot be made in a sufficiently short amount of time. PA1 (4) Information about the shape of flaw and the like is almost unavailable in the magnetic leakage flux technique. Discovery of a source therefore requires observation and investigation of the flaw, thus making the source discovering operation more complicated.
Of these two types of immersion testing techniques, the technique (2) inevitably takes the form of a batch testing, and for the practical online application on a production line of a strip, the technique (1) is more suitable.
The immersion ultrasonic testing techniques are classified in terms of the kind of the ultrasonic probe into a pulse-who technique using a transmitting/receiving probe, a pulse-who technique using a double crystal ultrasonic probe, and a transmission technique based on arrangement of transmitting probe and receiving probe face to face with a strip to be tested between them.
In general, however, the ultrasonic beam is focused into a spot ("spot focused," for example, with a diameter of 1 mm) in these techniques for increasing the detectability of the flaws to a sufficient level. Consequently, in these techniques, a large number of probes are required corresponding to the testing area. Thus, the number of the parts for the detecting instrument is large, which increases cost. The pulse-echo technique has a disadvantage of the presence of a dead zone directly below the surface of the strip.
In view of the disadvantages of the above-mentioned techniques, the present inventors proposed a testing method as disclosed in Japanese Unexamined Patent Publication Nos. 7-253414 and Japanese Unexamined Patent Publication No. 11-083815, that solves the problems involved in the test using above-described spot focused ultrasonic probe requiring many probes for testing the full volume.
The proposed method comprises conducting the pulse-echo testing by configuring the flaw detecting heads (hereinafter referred to as "detecting heads") in an immersion and transmission-type arrangement. The term "transmission-type arrangement" means arranging a transmitting head and a receiving head face to face with the strip to be tested between them.
This method comprises transmitting a line-focused ultrasonic beam in the thickness direction of the strip, and receiving a echo from the flaw with a receiving head including a probe array of piezoelectric elements arranged in the width direction of the strip to be tested.
More specifically, the transmitting head comprises line-focused transmitting probe arrays arranged in the width direction of the strip to be tested, and the receiving head comprises line-focused receiving probe arrays arranged also in the width direction of the strip to be tested. The transmitting probe array and the receiving probe array are arranged face to face on the opposite side of the strip. Part of ultrasound transmitted with the transmitting probe arrays is reflect at the flaw and is received with the receiving probe arrays faced transmitting probe arrays (see FIGS. 3 and 4).
The method permits detection of flaws, if any, in the strip to be tested without a dead zone directly below the top surface and the bottom surface by using the configuration as described above. The detecting head having this configuration is hereinafter referred to as an "ultrasonic line sensor."
Functions of the ultrasonic line sensor are illustrated in FIG. 7(A), where:
Flaw echoes F1 and F2 appearing between the transmission waves T1 and T2 are passed by a gating circuit, and when F1 or F2 has a amplitude more than a predetermined threshold voltage, it is detected as a flaw.
The line-focused probe array, having a wide range of testing in the width direction covered by a single detecting head, is preferable for the detection of flaws in a strip in transfer.
When detecting a flaw using immersion testing apparatus, the liquid (water) immersing technique is important for maintaining a satisfactory coupling between the ultrasonic probe and the steel sheet, i.e., for improving the detecting reliability. The applicable conventional techniques include the waterjet technique disclosed in Japanese Unexamined Patent Publication No. 7-113795, and the water tank immersion technique using a sealing pinch roll disclosed in Japanese Unexamined Patent Publication No. 5-149929, as well as the technique disclosed in Japanese Unexamined Patent Publication No. 60-78345.
However, because there are the following problems (1) to (3) for the application of the water jet technique disclosed in Japanese Unexamined Patent Publication No. 7-113795 to continuous testing over the entire volume of the strip, it is considered desirable to apply the immersion testing technique using a liquid tank.
(1) The necessity of arranging many probes in the width direction of the strip requires many nozzles for forming a water jet, many parts, and a more complicated apparatus. This requires a higher equipment cost and a complicated maintenance operation. The many nozzles increase the probability of occurrence of malfunctioning nozzles, and tend to cause a decrease in reliability of the apparatus.
(2) While it is conceivable to house a plurality of probes in a single nozzle, the increase in the size of the waterjet causes the force of the water to diffuse and exceed the surface tension of the waterjet, thus making it impossible to form a stable waterjet.
(3) Collision of water causes more serious fluctuations of the height of the carrying path of the rolled metallic sheet, irrespective of the number of probes, accordingly leading to deterioration of the detecting reliability. If the rolled metallic sheet is separated more from the nozzle to avoid this, it becomes impossible to form a water jet.
The water tank immersion technique using a sealing pinch roll disclosed in Japanese Unexamined Patent Publication No. 5-149929 has, on the other hand, an advantage of permitting testing without changing the height of the carrying path of the rolled metallic sheet. According to an investigation carried out by the present inventors, however, application of this technique to automatic ultrasonic testing of a rolled metallic sheet causes the following problems when the carrying speed of the sheet is higher and ultrasonic testing requires a higher sensitivity.
(1) Bubbles tend to easily go into the gap between the ultrasonic probes and the rolled metallic sheet. These bubbles generate a bubble echo as shown in FIG. 7(B), and this echo may be falsely interpreted as a flaw echo. The propagation of the flaw echo may be interrupted by the bubbles, thus it will be impossible to accomplish detection of the actual internal flaw. According to an investigation carried out by the present inventors, generation of bubbles is caused by entrainment of air by rotation of the upper sealing roll partially exposed on the water surface.
(2) Because of a large quantity of water outflow at the gap of rolls, it is necessary to feed a large amount of water into the water tank. Gaps corresponding to the thickness of the rolled metallic sheet are produced at the axial ends of the upper and lower pinch rolls (portions not in contact with the running rolled metallic sheet), and the amount of leakage water from these portion cannot be disregarded.
Japanese Unexamined Patent Publication No. 5-149929 and Japanese Unexamined Patent Publication No. 60-78345 disclose a method of immersing a rolled metallic sheet into water in a water tank by changing the carrying path of the rolled metallic sheet with deflector rolls.
However, a study conducted by the present inventors revealed that this method also has a problem of bubbles produced in water in the water tank as in the above-mentioned problem (1). According to a study carried out by the present inventors, a main source of bubble production is that, when the strip moves substantially vertically from the water upward, water adhering to the strip and coming up from the water surface, drops in a large quantity (a quantity almost exponentially proportional to the carrying speed of the strip) onto the water surface. Bubbles produced through this mechanism are entrapped into the water flow in the water tank, diffused throughout the entire tank, and cause the same problem as in (1) above.
It is conceivable that a large distance between the deflector rolls in the water tank and the water surface would correspond to a smaller possibility of producing bubbles by the water flow chiefly caused by these rolls. Naturally, however, this distance must be designed with a value larger than an anticipated one because there are no available guidelines for a specific value of the distance. Particularly, a design made based on the anticipation of high-speed would result in a deep water tank, and larger-scale equipment would be needed.
If the above problems are solved and it becomes possible to continuously test a carried strip by ultrasonic immersion testing technique, application of the technique to a manufacturing process of, for example, a steel sheet (steel strip) would be conceivable.
It is the conventional practice to perform continuous online testing of a cold-rolled steel sheet by the magnetic leakage flux testing technique on a production line after cold rolling, such as a finishing process. This testing is performed on a production line after cold rolling because determination of shipping can be made by testing carried out immediately before product shipment.
According to considerations made by the present inventors, however, the practice of testing on a production line after cold rolling has the following problems:
In the manufacture of a hot-rolled sheet, on the other hand, it is believed to suffice to provide quality assurance by testing through a sampling inspection of product hot-rolled steel sheets. The necessity or advantage of flaw investigation for the full width and full length as in cold-rolled steel sheets has never been established. Actually, testing apparatus permitting a high-accuracy, full width-full length testing at a high speed for hot-rolled steel sheets has not practically been provided until recently.
Even for hot-rolled steel sheets, however, there is an increasing demand from users for conducting operations of a working line or the like at a high efficiency without problems. It is therefore considered desirable to provide quality assurance based on total testing rather than only a probability assurance.